Thermotropic liquid-crystalline polyester resin (which is called liquid-crystalline polyester resin or LCP hereinafter) has good properties including heat resistance, mechanical properties such as rigidity, chemical resistance and dimensional accuracy and is used not only for molded articles but also for a variety of products such as fibers and films. Particularly, personal computers and mobile phones employ highly integrated devices and the art wishes to use downsized, thinner and smaller parts for them. In the information and telecommunication fields, very thin parts, as thin as 0.5 mm or less of the thickness, are sometimes required. Based on the excellent molding properties of the LCPs including good flowability and less flash development compared to the other thermoplastic resins, consumption of the LCPs has been increasing.
Liquid-crystalline polyester resins which have good mechanical properties, thermal properties and an excellent moldability have been proposed. For example, Japanese Patent Application Laid open No. Hei. 01-294732 and U.S. Pat. No. 4,849,499 (the cited references are herein incorporated by reference) provide liquid-crystalline polyester resins produced by polymerizing monomers such as p-hydroxybenzoic acid, terephthalic acid, 2,6-naphthalene dicarboxylic acid and 4,4′-dihydroxybiphenyl by a method such as slurry polymerization method or molten acidolysis.
Recently, liquid-crystalline polyester resins are employed for a variety of uses wherein the LCPs are processed at a high-temperature such as solder-welding and infrared reflow-welding. However, the liquid-crystalline polyester resins produced by polymerization a method such as slurry polymerization or molten acidolysis not exhibit sufficient heat resistance.
In order to obtain a liquid-crystalline polyester resin with an excellent heat resistance, methods using solid phase polymerization have been proposed. For example, U.S. Pat. No. 6,582,625, Japanese Patent Application Laid Open No. 2000-248056 and Japanese Patent Application Laid Open No. Hei. 11-246654 (the cited references are herein incorporated by reference) disclose methods for producing liquid-crystalline polyester resins which comprise preparing powdery or pellet-formed prepolymer with a low polymerization degree and heating said prepolymer in solid phase to give polymers with high degree of polymerization.
As liquid-crystalline polyester resins obtained by said solid phase polymerization method, U.S. Pat. Nos. 5,015,723, 4,742,149, 4,639,504 and 4,311,823 (the references are herein incorporated by reference) disclose liquid-crystalline polyester resins composed of following monomers:    i) p-hydroxybenzoic acid,    ii) terephthalic acid,    iii) 4,4′-dihydroxybiphenyl,    iv) 2,6-dihydroxynaphthalene, and    v) aromatic diols such as hydroquinone.
In addition, Japanese Patent Application Laid Open No. Sho. 62-235321 (the cited reference is herein incorporated by reference) discloses a liquid-crystalline polyester resin composed of following monomers:    i) p-hydroxybenzoic acid,    iii) 4,4′-dihydroxybiphenyl,    iv) 2,6-dihydroxynaphthalene,    v) aromatic diols such as hydroquinone, and    vi) 2,6-naphthalene dicarboxylic acid.
Conventionally known liquid-crystalline polyester resins obtained by solid phase polymerization exhibit good heat resistance because of high degree of polymerization. However, such liquid-crystalline polyester resins have a relatively high melt point and high melt viscosity and therefore, their molding processability was not satisfactory. The art desires liquid-crystalline polyester resins which show both high heat resistance and excellent moldability.
The art also desires liquid-crystalline polyester resins containing no flame retarder such as harmful bromine compound which can exhibit good flame retardancy.